Many kinds of add and drop frequency multiplexers or switching systems of the above type are known in the art.
In particular, FIG. 1 shows diagrammatically a system of the above kind for multiplexing N channels. That system is described in “Integrated Multichannel Optical Wavelength Selective Switches Incorporating an Arrayed-Waveguide Grating Multiplexer and Thermooptic Switches”, April 1998, J. of Lightwave Technol., Vol. 16, No. 4, pp. 650-655.
The system shown in FIG. 1 includes an arrayed waveguide grating (AWG) 10. The AWG 10 has 2N+2 input ports numbered 1 to 2N+2 and 2N+2 output ports also numbered 1 to 2N+2. An array of N 2×2 optical switches SW1 to SWN interconnects the input and output ports of the AWG 10 via looping lines 20.
A first N-channel input multiplexer M1 codes information i1, i2, . . . , iN on respective frequencies f1, f2, . . . , fN and a second N-channel input multiplexer M2 codes information i′1, i′2, . . . , i′N on the respective frequencies f1, f2, . . . , fN.
Thus the same frequencies f1 to fN convey different information. The system switches information coded on one or more particular frequencies between the two input multiplexes and the two multiplexes recovered at the output of the system.
In the FIG. 1 example, it is typically required to switch information i2 coded on the frequency f2 and information iN coded on the frequency fN with information i′2 and i′N coded on the same frequencies between the two input ports and the two output ports of the system.
The operating principle for this particular example is based on the fact that the frequencies of the first multiplex that enter at the input port 1 are to be demultiplexed to the output ports N+3 to 2N+2 and all the frequencies of the second multiplex that enter at the input port N+2 are to be demultiplexed to the output ports 2 to N+1.
The signals demultiplexed in this way are then guided to the N 2×2 optical switches SW1 to SWN. The signals with the same frequency fi from the two input multiplexes M1 and M2 are sent to the same 2×2 switch SWi. The output signals switched by the N 2×2 switches are then looped to the input ports of the AWG 10.
The signals looped to the input ports N+3 to 2N+2 and the signals looped to the input ports 2 to N+1 are automatically re-multiplexed and sent to AWG output ports 1 and N+2, respectively.
For a particular frequency fi, each switch SWi sends the signal coded on the frequency fi either to the first set of input ports 2 to N+1, to be more precise to the input port i+1, or to the second set of input ports N+3 to 2N+2, to be more precise to the input port i+N+2. Each switch SWi therefore changes the output port numbered 1 or N+2 to which the information coded at the frequency fi is to be sent.
However, the routing element employed in the FIG. 1 system, i.e. the AWG, is not optimized at all in terms of the number of channels. Processing N channels with the above kind of architecture requires a routing element capable of routing 2N+2 channels. The AWG is therefore rated higher than is strictly necessary, requiring 2N+2 input ports and 2N+2 output ports to process N frequencies.
This amounts to approximately doubling the number of waveguides in the AWG, making the system complex and costly to implement.
Another drawback of the above solution is that the system is based on the use of 2×2 optical switches.
If the switches are thermo-optical switches, the operating speed of the system is limited. With thermo-optical switches, the system is incapable of selecting frequency-coded information in less than a few nanoseconds.
Accordingly, the object of the present invention is to provide a compact and fast system which is capable of multiple switching of frequency division multiplex signals between two input ports and two output ports and which alleviates the drawbacks of the prior art.
To this end, the invention exploits the routing properties of AWG multiplexers to expand the architecture of a conventional wavelength selector to yield an architecture comprising a plurality of interleaved stages of optical switches.
The incoming frequency division multiplex optical spectra are therefore divided by a first demultiplexer and sent to a plurality of interleaved stages of optical switches which selectively feed a plurality of input ports of a multiplexer whose routing properties are then used in particular to implement the complex function referred to above, namely reconfigurable multiple 2×2 frequency switching.
It is also proposed, starting from the architecture of the switching system according to the present invention, to use an optical delay system able to adopt multiple configurations and to store, delay and extract variable length optical packets.
Optical delay circuits of the above kind are known in the art, in particular the solution developed by NTT and described in “Variable optical delay circuit using wavelength converters”, March 2002, Electron. Lett., Vol. 37, No. 7, pp. 454-455. FIG. 2 is a diagram of the above kind of prior art system, which is described next with reference to that figure.
A frequency fj conveying information ij arrives at an optical input port 1 of the system. In the FIG. 2 example, j is from 1 to 5 and the optical packets can therefore be coded on five different frequencies. Consider the case where an optical packet is coded on the frequency f1. A coupler 2 sends the signal to a circulator 3 which directs the signal to an AWG demultiplexer Demux. A filter 4 is inserted between the circulator 3 and the AWG Demux to reject the frequency f5. Because the packet is coded at f1, it is not rejected by the filter 4 and is therefore demultiplexed and sent to the first output line of the demultiplexer Demux in accordance with a conventional routing property of the AWG. Each of the four output lines of the demultiplexer Demux for respectively receiving the signal coded on f1, the signal coded on f2, the signal coded on f3 and the signal coded on f4 has a first stage comprising a semiconductor optical amplifier 5, a second stage comprising an optical band-pass filter 6, and finally a final stage comprising a wavelength converter 8, one input of which is connected to a laser diode 7.
Accordingly, circulation losses are compensated by the optical amplifiers and unwanted noise introduced by the amplifier is eliminated by the optical filter 6. The optical converter 8 on the far side of each filter 6 is fed with the demultiplexed signal and with the signal from one of the laser diodes 7 for the respective four lines, which supply the converters 8 with the frequencies f2, f3, f4 and f5.
Accordingly, in the present example of a packet initially coded on f1, the packet is then coded on f2. The signal is re-multiplexed by the multiplexer Mux and fed into the optical delay loop 9. It therefore passes again through the coupler 2, the circulator 3 and the filter 4. Because the signal is now coded on f2, it is not rejected by the filter. This time it is demultiplexed onto the second output line of the demultiplexer Demux and therefore converted to the frequency f3 before it is fed again into the optical loop, and so on.
Accordingly, the wavelength of the optical signal inserted into the loop is sequentially shifted until it reaches the frequency f5. Once it has reached the frequency f5, the signal is rejected by the filter 4 and is sent to the output of the delay circuit via the circulator 3.
In this example, the packet initially coded on f1 is inserted four times into the delay loop before it is converted to the frequency f5. In the same way, a signal arriving in the delay circuit coded on f2 travels round the loop three times, a signal coded on f3 travels round the loop twice, and so on, until a signal coded on f5 is not looped at all.
Consequently, the number of times the signal travels round the loop, or to be more precise the time-delay introduced by the delay loop prior to the release of the signal, has a value that depends directly on the frequency on which the signal is initially coded.
The characteristic relating the duration of the time-delay to the frequency on which the signal is coded is itself a limiting factor on the architecture, since no flexibility is allowed in terms of the duration of the time-delays. Accordingly, it is not possible to adjust the delay at will to a required duration, as the duration is directly dependent on the frequency on which the signal is coded.
Moreover, the above prior art solution imposes the use of many somewhat complex elements, in particular the combination of the laser diodes 7 and the wavelength converters 8. The complexity of this solution therefore constitutes a brake on the implementation of this kind of optical delay circuit.